The two (02) components used are bitumen obtained from Arab Contractor’S Ltd, a public works company in Cameroon, and aggregates from the AKAK ESSE quarry, in the Mvila department, South Cameroon Region.

Three (03) partial bitumen substitutes are used, including sulphur, tyre powder and plastic bottle waste powder.

This consists of determining the following parameters in accordance with the relevant standards:

~ Moisture content (NF P 94 - 050);

~ Particle size analysis (NF P 94 - 056);

~ Los Angelès coefficient (NF P 18 - 573);

~ Micro Deval coefficient (NF P 18 - 572);

~ Flattening coefficient (NF P 18 - 561);

~ Sand equivalent (NF P 18 - 598).

1) Formulation and manufacture of control asphalt concrete

The formulation consists of determining the rate of each granular component in accordance with standard NF P 98-150-1, and then determining the percentage of bitumen (P_bi) calculated using Equation (1) [9] .

P b i = ( T G + 120 ) / 100 (1)

where TG, the total particle size, is the sum of the percentages of passings of the granular mix at mesh sizes 16, 12.5, 10, 6.3, 4, 2 and 0.080.

The control asphalt concrete is manufactured in accordance with standard NF EN 13 108-1.

2) Characterization of the control asphalt concrete

This is carried out by means of level 2 mix design tests which are:

~ PCG test (NF EN-P 12 697 - 31);

~ Duriez test (NF P 98 251 - 1);

~ The rutting test (NF EN-P 12 697 - 22).

They are produced using sulphur, tyre powder and plastic bottle powder.

1) Obtaining partial substituent of bitumen

Three (03) partial bitumen substituents will be used, namely sulphur, tyre powder and plastic bottle powder. They will be obtained following a very precise procedure.

The sulphur was bought on the local Cameroonian market in the form of carrots before being crushed and sieved through a 1-millimetre mesh sieve to make it easier to weigh. Extracted in neighbouring Nigeria, the carrot shape of the sulphur indicates that it was obtained using the Flash process.

The powdered plastic bottle waste will be obtained in eight (08) consecutive stages, including:

a) Collection of waste plastic bottles (non-opaque, transparent and coloured);

b) Removal of the caps;

c) Cutting into flakes and manual washing;

d) Spinning and drying;

e) Melting;

f) Cooling the paste to room temperature;

g) Manual grinding of the hardened paste;

h) Screening with a 1 mm sieve.

The tyre powder will be obtained in seven consecutive stages, including:

a) Collection of waste tyres;

b) Manual coarse cutting into flakes;

c) Cutting into elements of approximately 1 cm²;

d) Washing under running water;

e) Dewatering;

f) Shredding;

g) Screening with a 1 mm sieve;

h) Magnetic purification.

2) Formulation of modified bituminous concrete

Modified bituminous concretes are just 10%, 20%, 30% and then 40% bitumen by the sum by mass of tyre powder, plastic bottle powder and sulphur, at a rate of 40%, 28% and 32% respectively.

3) Manufacture of modified bituminous concrete

This is done using the wet process [10] and in accordance with the Guide [11] for the manufacture of asphalt mixes with rubber powder from used tyres in Spain.

4) Characterization of modified bituminous concrete

This is also done by means of level 2 mix design tests.

Two (02) Duriez parameters are presented in Table 4 including the simple compressive strength “R_C” and the degradation coefficient “α”.

It is observed that the simple compressive strength increases progressively with an increase of 12.5% for 40% substitution. This gap is identical to that of asphalt concrete modified by the addition of 15% plastics [12] . The strength of BBM₄₀ (10.4 MPa) is close to that of some anti-rutting bituminous concretes, in particular REXOVIA (10.2 MPa) [13] , and RUFLEX M (9.8 MPa) [14] produced

respectively by the laboratories EUROVIA (which is a VINCI company) [13] and COLAS. As a result, BBM₄₀ is classified as an anti-rutting bituminous concrete. Furthermore, BBM₄₀ is of lower strength than so-called very high-performance bituminous concretes such as ECOFLEX LT (11.3 MPa) [15] and MULTICOL (15 MPa) [16] produced respectively by the SCREG and COLAS laboratories. Therefore, BBM₄₀ is not one of the so-called very high-performance bituminous concretes.

All variants have a good ability to resist stripping under mechanical stress in the presence of water depending on the requirement. Only the BBM₄₀ has a coefficient close to that of the material whose degradation coefficient is 0.85 [17] . Also, it is improved that, this parameter is by 09% for asphalt concrete modified with 15% plastics whereas the reinforcement is only 01.21% for the present work [12] . Furthermore, the degradation coefficient of BBM₄₀ is slightly higher than that of REXOVIA (0.81) [13] while ECOFLEX LT (0.89) [15] , MULTICOL (0.9) [16] and RUFLEX M 0/10 (0.98) [14] withstand static mechanical stresses under heavy precipitation better than BBM₄₀.

The increase in simple compressive strength results from the formation of several linear and/or star-shaped bonds [18] [19] that become more intertwined followed by the development of hydrophilic character.

Table 5 shows the variations in voids and compactness, PCG parameters.

Examination of Table 5 reveals that only BBM₄₀ has a non-compliant voids rate per 100 gyrations, whereas this specification of standard NF P 98 - 252 is verified by the other variants.

Indeed, the compactness of BBM_i is higher than that of the material by 94% of compactness [17] . But the 01.04% improvement obtained for the present work is

lower than the 05% improvement recorded for bituminous concretes manufactured by dry process, obtained by partial substitution of bitumen by rubber powder and those from partial substitution of 0/3 by rubber powder [20] . The same is true for asphalt concretes doped with rubber fines and those doped with rubber granules [21] .

Furthermore, at 10 gyrations, the four (04) modified bituminous concretes are more cellular than the anti-rutting bituminous concretes, in particular ECOFLEX LT (11.5%) [15] and COFLEX N (12.6%) on the one hand, and on the other, the very high performance concretes, namely Shell Thiopave and MULTICOL [16] , which have a maximum of 11% voids. On the other hand, at 80 gyrations, the compactness of MULTICOL (93%) is lower than that of each BBM_i.

The increasing evolution of workability observed via the progression of compactness is, on the one hand, a consequence of the plastic bottle powder which reduces the cellular structure by melting and cooling [20] of the granular skeleton matrix and, on the other hand, a result of the double sulphuric cross-linking [19] [21] .

Changes in rutting resistance are shown in Table 6.

Resistance to rutting increases with the rate of substitution and the rut depth rate falls by 19% for 40% substitution. Similarly, another work [22] [23] shows that incorporating plastic into bitumen increases its rigidity at high temperatures and also boosts rutting resistance. This theory of improving resistance to rutting by injecting plastic into the bitumen is corroborated by Nouali [24] , while Zhang et al (2018) [25] reached the same conclusions by substituting 1.5% of aggregates with tyre powder.

Furthermore, rutting-resistant bituminous concretes such as REXOVIA [13] , ECOFLEX LT [15] and RUFLEX M [14] have rut depths of 5%, 3% and 5.6% respectively. These values, which are fairly close to those of BBM₃₀ (5.91%) and BBM₄₀ (5.90%), can be used to classify the last two (02) variants as anti-rutting bituminous concretes.

With a susceptibility to rutting of 7.5% at 30,000 cycles, class 2 high modulus asphalt is not preferable to BBM₄₀ (5.9%), which is less prone to rutting than MULTICOL [16] , which has a maximum rut depth of 3%.

The improvement in the stability of the material under the combined action of dynamic and thermal stresses derives from the interactions between the sulphur and the plastic bottle powder and then between the sulphur and the tyre powder [18] [19] .